Hi Wiliamseal, thanks for your kind comment.
Swapping B3 into the top row allows the Roman version (6 letters) of Leto to appear. Perhaps I was quick to be critical in an earlier comment, since I now see on the wikipedia page for Niobe, that it was (according to Homer) Leto who punished Niobe by sending Apollo and Artemis to kill all her children. If she had enough influence and power to do this, then probably Leto could have been responsible herself (and not requiring the power of Zeus) for turning Niobe to stone. So that accounted for Niobe.
Then you have the character who (after swapping in the A answers) appears instead of Niobe. In this case, wikipedia is quite explicit that this character was turned to stone by Perseus, who appears at the opposite corner of the grid from Leto after swapping in the B answers.
I hope this helps, and best wishes for the New Year.